Direct chill casting is a process in which molten metal is fed to a water-cooled mould and a cast metal strand is withdrawn from the mould. Direct chill casting includes horizontal direct chill casting and vertical direct chill casting. The strand withdrawn from She mould consists of a solid shell surrounding a liquid core which subsequently solidifies. The strand is sprayed with cooling water as it exits the mould to both cool the strand and to extract additional heat from within the water-cooled mould. A casting lubricant is used in direct chill casting to reduce friction between the solidified shell and the face of the mould for the deal purposes of reducing mould wear and avoiding tearing of the shell.
Casting lubricants include casting oils and casting greases. Casting oils are typically continuously fed to the interface between the face of the mould and the metal by an arrangement which is either integrally formed with the mould or which is separately constructed to function with the mould. Casting greases may be applied manually by brushing the face of the mould prior to casting or by use of an automatic grease lubricating system such as that disclosed in International Publication No. WO 94/00258. casting lubricants require high thermal stability and resistance to oxidative degradation at high temperatures. Throughout this specification, the expression "casting lubricant" is to be understood to include such casting oils and greases and includes animal, vegetable, synthetic and mineral casting oils and greases such as vegetable shortening, lard used in baking, castor oil, rape seed oil, esters, paraffins and synthetic liquids.
Magnesium is a highly reactive and thermodynamically unstable element with molten magnesium readily oxidizing in ambient air. Three approaches have been used to inhibit the severe oxidation process. Salt cover fluxes may be sprinkled over the molten metal; oxygen may be excluded from contacting the molten metal by blanketing the molten metal with an inert gas such as helium, nitrogen or argon; or a protective cover gas may be used to blanket the molten metal. Protective cover gases typically comprise air and/or carbon dioxide and a small amount of an inhibiting agent which reacts/interacts with the molten metal to form a film/layer on the molten metal which protects it from oxidation. The mechanism by which inhibiting agents protect molten reactive metals is not well understood.
U.S. Pat. No. 1,972,317 was filed in 1932 and teaches a method for inhibiting the oxidation of readily oxidizable metals (for example, magnesium) by maintaining a fluorine containing atmosphere in contact with the surface of the metal. Lines 70-74 of the patent state, "The present invention is not concerned with, nor do the claims cover, any step in which the inhibition of detrimental attack by atmospheric gases upon magnesium takes place within a casting mould."
U.S. Pat. No. 3,034,186 was filed in 1956 and teaches an aluminum casting lubricant consisting of a dispersion of solid boric acid (H.sub.3 BO.sub.3) in a suitable oil or oily based material. The casting lubricant is said to inhibit metal oxidation on ingot surfaces and prevent magnesium burning in aluminum alloys with high magnesium content. The casting lubricant is believed to have never been commercially used.
U.S. Pat. No. 4,930,566 teaches a method for continuous casting of an aluminium-lithium alloy through an open-ended mould which is lubricated by a casting lubricant. Pressurized gas is directed into the mould to contact the molten metal immediately prior to solidification to displace the point at which tile metal contacts the mould. The gas comprises 1-15% by volume oxygen with the balance being an inert and incombustible gas such as nitrogen, argon, carbon dioxide, helium or a mixture thereof. A variation of the teaching of U.S. Pat. No. 4,930,566 relates to the casting or magnesium in which the pressurized gas is carbon dioxide containing 10% sulphur hexafluoride (SF.sub.6). The gas and the casting lubricant are separately introduced into the mould.
CA 2047384 reaches a method for vertical direct chill casting in which a casting lubricant is used to lubricate the mould and a protective gas containing a high concentration of an inhibiting agent such as SF.sub.6 is drawn into the mould by a self-generating vacuum.
JP 2-277098 teaches a non-ferrous metal casting lubricant consisting of a plant oil lubricant containing 0.5-3% by weight ot powdered boron nitride which is said to improve lubricating properties and reduce the amount of lubricant required as compared with plant oil lubricant alone.